The invention relates in general to analog data processing systems and more particularly to low level analog data acquisition multiplexed subsystem interfaced with a digital subsystem.
Digital computer process control or monitoring requires many input data in analog form which are derived by sensing devices located at various points of the controlled industrial process. These input data are converted into digital form for fast and complex treatment within a digital processing system before output signals can be supplied for control or monitoring action. A digital system includes digital circuitry which controls the opening and closing of relays associated with the analog multiplexing system. As a result of such relay closing and opening actions, input data acquisition is obtained from the multiplexed system through cables providing communication between the interface of the digital system and the various and remote locations where the transducers sense process variable instantaneous conditions. The multiplexed cable system leads to terminals which form the input to an analog digital converter providing the necessary transformation from analog to digital of the analog signals sequentially received each during a time interval of measurement generally of 1/60 of a second duration. Since the sensing devices are remotely located, long cables are necessary for the transmission of the measurement signals. These signals, however, have a small magnitude, so that the cables have to be shielded from parasitic interference to minimize noise, and these shields are connected electrically to form a potential floating system from the point of measurement to the input of the analog digital converter.
Problems arise due to common mode, normal mode potentials, and offset voltages, which potential and voltages may often exceed by several orders the magnitude of the sensed analog signal. These problems are more difficult to cope with when high speed multiplexing is involved. In particular high voltage switches are required to stand off common mode voltages, and offset voltage errors subsist. Attempts have been made in the prior art to solve these problems in general. For instance, it is known to modulate each of the input signals received on a plurality of channels in order to take advantage of transformer coupling and to allow the associated channel to float freely on one side of the transformer according to a generally known technique for rejecting the common mode noise. However, such prior art solutions do not eliminate the errors due to offset voltages and contact potentials. These errors subsist after demodulation in the input signal reading effect at the interface with the digital system.
It is known from U.S. Pat. No. 2,877,308 of Reiner to periodically inverse both the input and the output connections of a direct current integrator having a balanced amplifier in order to eliminate offset errors in the reading of low-level analog signals. However, the Reiner patent does not deal with these problems in the context of a multiplexed analog signal system coupled to a digital system.
It is known, from U.S. Pat. No. 3,656,065 of Reinhard, to transmit a low-level analog imput signal from a floating data acquisition system including a floating input differential amplifier, through an isolation power transformer after modulation by an AC signal and to synchronously demodulate the signal for reconversion as one analog signal to be read. In this arrangement, however, modulation is effected after the input differential amplifier and it does not eliminate potential offset errors.
The object of the present invention is to provide an improved multiplexed analog data acquisition system suitable for low level input signals and capable of being interfaced with a digital system.
Another object of the present invention is to provide an improved analog signal processing system which is compatible with digital processing techniques and which is free from offset voltage errors and contact potentials as generally encountered when dealing with low level input signals derived from remote locations.